Electronic devices, especially portable electronic devices, typically utilize low operation voltage in order to reduce heat generation and power consumption. Increasingly, portable electronic devices are combining traditional functions with additional functions. For example, some portable phones include a camera function for still images and a camcorder function for moving images. High capacity storage is typically required in such electronic devices in order to smoothly process both the traditional and additional functions.
As is well known, NAND flash memory is suitable for a portable electronic device as a high capacity storage. As such, NAND flash memory is traditionally packaged into a detachable card that can be inserted into or detached from a portable electronic device according to user's demand. For example, a detachable card such as a multi-media card (MMC), a secure-digital card (SD card), a smart-media card and a compact-flash card can be used as storage for voice, image and data in a digital camera, an MP3 player, a PDA, a handheld PC, a video game, a fax, a scanner, a printer, etc.
Detachable cards used as portable storage devices can encounter various operating situations. For example, a detachable card can be used in an electronic device operating at an operation voltage (VDD) of 3.3V. A detachable card can be used in an electronic device operating at a voltage lower than the operation voltage (VDD) of 3.3V. The normal operation of a card cannot be guaranteed when a card operating at the operation voltage (VDD) of 3.3V is used in a portable electronic device operating at a low operation voltage. A card designed to operate with the operation voltage (VDD) of 3.3V can deteriorate the low voltage characteristic.
Accordingly, a lock-out device is used to preserve the low voltage characteristic of electronic devices. The lock-out device terminates a mode in operation and executes a mode recovery when the operation voltage (VDD) becomes lower than a predetermined voltage level. A lock-out device and a system equipped with such a lock-out device are disclosed in U.S. Pat. No. 6,434,653, issued on Aug. 13, 2002 to Winston, entitled “METHOD AND APPARATUS FOR DISABLING POWER-ON IN A SYSTEM REQUIRING ADD-IN MODULES”. As is well known to those skilled in the art, a lock-out device is generally applied to a detachable card system, a single-chip type flash-memory device and various types of semiconductor devices equipped with flash memory. Moreover, a lock-out device is applied to not only the flash memory but also to the integrated circuit device equipped with various types of memory devices.
FIG. 1 is a timing diagram of a lock-out device and exemplifies the operation of a flash memory operating at a high voltage.
In FIG. 1, the lock-out circuit produces a lock-out control signal activated to a high level when the operation voltage VDD drops down below a predetermined level Vdetect by noise and so forth during the operation of an integrated circuit. The integrated circuit device terminates a program operation or an erase operation in response to the activated lock-out control signal VLKO and executes a mode recovery.
In general, one integrated circuit is equipped with one lock-out circuit. Therefore, the voltage-drop result of the operation voltage VDD detected from the lock-out circuit reflects the result of voltage drop in a partial chip rather than the result of voltage drop in an entire chip. The condition of terminating the program operation or the erase operation can be changed according to the arrangement of the lock-out circuit in a chip. For example, since only a pump circuit operates while a high voltage is set up during the program operation or the erase operation in the flash memory, the drop in some degree of the operation voltage VDD is not a great issue. Nevertheless, since the lock-out circuit arranged near the pump circuit is influenced from the operation of the pump circuit, there sometimes occurs the unnecessary termination of operation.